1. Field of the Invention
This invention relates generally to motorized riding trowels for finishing concrete surfaces of the type classified in United States Patent Class 404, Subclass 112. More particularly, our invention relates to multiple-rotor, hydraulically driven riding trowels.
2. Description of the Prior Art
It is well established in the concrete finishing arts that freshly placed concrete must be appropriately finished to achieve the desired smoothness and flatness. As freshly poured concrete "sets", it soon becomes hard enough to support the weight of motorized riding trowels, that are particularly effective for finishing concrete. Motorized riding trowels are ideal for finishing large areas of plastic concrete quickly and efficiently, and a variety of riding trowels are known in the art.
Typical riding trowels employ multiple, downwardly projecting rotors that contact the concrete surface and support the weight of the trowel. A typical rotor comprises a plurality of radially spaced apart finishing blades that revolve in frictional contact with the concrete surface. The blades may be coupled to circular finishing pans for treating green concrete. When the rotors are tilted, steering and propulsion forces are frictionally developed by the blades (or pans) against the concrete surface. Riding trowels finish large surface areas of wet concrete more efficiently than older "walk behind" trowels. Significant savings are experienced by the contractor using such equipment, as time constraints and labor expenses are reduced.
Preferably, the finishing process starts with panning while the concrete is still "green", within one to several hours after pouring depending upon the concrete mixture involved. The advent of more stringent concrete surface finish specifications using "F" numbers to specify flatness (ff) and levelness (fl), dictates the use of pans on a widespread basis. Both "super-flat" and "super-smooth" floors can be achieved by panning with motorized trowels.
Pan finishing is normally followed by medium speed blade finishing, after the pans are removed from the rotors. A developing technique is the use of "combo blades" during the intermediate "fuzz stage" as the concrete continues to harden. So-called "combo-blades" are a compromise between pans and normal finishing blades. They present more surface area to the concrete than normal finishing blades, and attack at a less acute angle. The rotors are preferably turned between 100 to 135 RPM at this time. Finishing blades are then used, and they are rotated between 120 to 150 RPM. Finally, the pitch of the blades is changed to a relatively high contact angle, and burnishing begins. Rotor speeds of between 135 and 165 RPM are recommended in the final trowel finishing stage.
Holz, in U.S. Pat. No. 4,046,484 shows a pioneer, twin rotor, self-propelled riding trowel wherein the rotors are tilted to generate steering forces. U.S. Pat. No. 3,936,212, also issued to Holz, shows a three rotor riding trowel powered by a single motor. Although the designs depicted in the latter two Holz patents were pioneers in the riding trowel arts, the devices were difficult to steer and control.
Prior U.S. Pat. No. 5,108,220 owned by Allen Engineering Corporation, the same assignee as in this case, relates to an improved, fast steering system for riding trowels. Its steering system enhances riding trowel maneuverability and control. The latter fast steering riding trowel is also the subject of U.S. Des. Pat. No. 323,510 owned by Allen Engineering Corporation.
U.S. Pat. No. 5,613,801, issued Mar. 25, 1997 to Allen Engineering Corporation discloses a power-riding trowel equipped with separate motors for each rotor. Steering is accomplished with structure similar to that depicted in U.S. Pat. No. 5,108,220 previously discussed.
Allen Engineering Corporation U.S. Pat. No. 5,480,258 discloses a multiple engine riding trowel. The twin rotor design depicted therein associates a separate engine with each rotor. As the engines are disposed directly over each revolving rotor assembly, horsepower is more efficiently transferred to the revolving blades. Besides resulting in a faster and more efficient trowel, the design is easier to steer. Again, manually activated steering linkages are used.
Allen Engineering Corporation U.S. Pat. No. 5,685,667 discloses a twin engine riding trowel using "contra rotation." Many trowel users prefer the steering characteristics that result when the trowel rotors are forced to rotate in a direction opposite from that normally expected in the art.
While modern, high power riding trowels are noted for their speed and efficiency, extreme demands are placed upon the relatively small, internal combustion motors that power such machines. Adequate horsepower must be available at all times for the rotors, that must operate under varying conditions of speed, drag, rotor tilt-angle, blade pitch, and concrete hardness. Demands upon drive motors can vary widely when switching between panning and blade-finishing modes. Generally speaking, the more powerful the trowel, the faster finishing operations can be completed. However, optimum engine speed (i.e., for rated torque and horsepower) is limited to a relatively small RPM range. On the other hand, a variety of blade speeds are required for modern finishing, and as explained earlier, load conditions vary widely as well. Engine RPM is usually the key variable related to output power. Typical riding trowel engines are coupled through belts and pulleys to gear boxes connected to the rotor shafts. The output shaft speed (i.e., rotor speed) is geared down, with a ratio of 20:1 being common. While it is recognized that effective motor output characteristics are RPM related, the use of fixed ratio reduction gearing often results in a mismatch between the desired blade speed, the frictional load, and the available motor horsepower at a given RPM.
If engine speed increases too much, excessive power may be developed, and the finishing mechanism may rotate too fast. For example, the initial panning stage requires relatively high power because of the viscous character of green concrete, but relatively low rotor speeds are desired. Since the rotors are driven through a fixed ratio established by the gearbox, belts and drive pulleys, optimum engine power often cannot be obtained during panning without risking excessive rotor speeds.
It is thus desirable to provide a riding trowel wherein the engine and gear boxes can operate at ideal speeds over a wide range of finishing conditions. One solution pioneered by Allen Engineering Corporation, is the subject of pending U.S. patent application Ser. No. 09/008,355, filed Jan. 16, 1998, and entitled "Riding Trowel with Variable Ratio Transmission." The object is to vary the overall drive gear ratio during different panning and blade finishing stages so that motors may operate within optimum RPM ranges as much as possible. In the Allen design, the effective drive ratio established between the motor output pulleys and the drive pulleys splined to the gearbox input shaft can be dynamically varied. However, since the rotor gearbox reduction ratio is still fixed, the range of adjustment of the overall drive train gear ratio (i.e., the ratio between motor RPM and rotor RPM) is limited. What appears necessary is a variable ratio "drive gear" for revolving the rotors that allows the motors to maintain a relatively constant speed over a variety of working conditions and loads. Although hydraulic motors would seem logical, their practicality has hitherto been limited by the steering and handling characteristics of motorized trowels, and the available engine horsepower.
Many early riding trowels use manually operated levers for steering. The steering levers project upwardly from the frame and are grasped and manipulated by the operator to direct the machine. The steering levers deflect linkages below the trowel frame to tilt the rotors. Often a vigorous physical effort is required. Where separate engines are used with each rotor assembly, additional physical effort is required to tilt the rotors for steering, or to vary blade pitch. It has now been established that modern, state-of-the art riding trowels require power steering for maximum performance. Hydraulic steering systems for multiple engine trowels previously proposed by Allen Engineering Corporation have proven desirable. For example, copending Allen Engineering Corporation patent application Ser. No. 08/784,244, filed Jan. 15, 1997, entitled "Hydraulically Controlled Riding Trowel" discloses a powered steering system for riding trowels. Quick, responsive handling optimizes trowel efficiency, and preserves operator safety and comfort.
At the same time, power steering requires added hydraulic motors and accessories that increase the demand for motor horsepower. Hydraulic steering devices consume energy, further aggravating the need for power and optimal motor control. In other words, internal combustion engine drive speed should be maintained within an optimal RPM range to supply adequate horsepower. But, as explained earlier, the overall drive train gear ratio limits motor performance. By using hydraulic motors to drive trowel rotors, the internal combustion motors may operate continuously within ideal RPM ranges. The resultant horsepower increase more than offsets losses caused by hydraulic inefficiencies. Concomitantly, the added weight resulting from hydraulic drive motors and required accessories further burdens the steering system. The heavier and more powerful the trowel, the more important it is to establish responsive steering and fast, effective handling.
Hence we have designed a multiple-rotor, hydraulically driven trowel. In the best mode the hydraulic drive system is employed with an optimized steering control system.